1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an electrophotography method such as a copying machine or printer and, particularly relates to an image forming apparatus that cleans up residual toner on an intermediate transferring member, etc.
2. Related Background Art
Conventionally, various methods, such as electrophotography methods, thermal transfer methods and ink jet methods are well known for use in a color image forming apparatus. Among these, electrophotography methods have advantages over other methods in terms of the image forming rate, image quality and acoustical property.
Many methods exist for an image forming apparatus utilizing electrophotography. These include, for example, the multi-development method in which an image formed of a color developer (multicolored toner image) is put onto a surface of a photosensitive member and then it is transferred at a time onto a transfer material as a recording material, the multi-transfer system that repeats development-transfer cycles, and the intermediate transfer method in which toner images of each color are primarily transferred sequentially onto an intermediate transferring member and then they are transferred at a time onto a transfer material.
Among them, the intermediate transfer method particularly has advantages in that it does not cause mixing of colors, and that it allows for the use of various transfer materials with different qualities and thicknesses.
As an example of the image forming apparatus utilizing the intermediate transfer method which is the background art of the present invention, FIG. 5 shows schematically a four-color, full-color laser beam printer that uses an intermediate transferring belt 9 in the form of a belt as the intermediate transferring member.
As shown in FIG. 5, over the peripheral surface of a photosensitive drum 1 as an image bearing member, a charger 2, an exposing apparatus 3 that irradiates the photosensitive drum 1 with laser light, rotary 22, as a rotating unit, that is rotatably and movably provided with four developing apparatuses 5, 6, 7 and 8 each containing a developer of different color, the intermediate transferring belt 9, and a photosensitive drum cleaner 19 as means for cleaning the image bearing member are sequentially arranged along the rotational direction (in the arrow R1 direction) of the drum.
As shown in FIG. 5, the photosensitive drum 1 having a diameter of 46.7 mm, which is rotationally driven at a surface velocity of 117 mm/sec in a direction indicated by an arrow R1, is negatively charged by the charger 2 on its surface. The photosensitive drum 1 charged by the charger 2 typically has a surface potential (hereinafter referred to as a “charge potential”) from −450 V to −800 V. The charger 2 is applied with a charging bias that is an alternating voltage having a DC voltage superimposed on it with a charger power supply 17.
An electrostatic latent image is formed on the surface of the charged photosensitive drum 1 by an exposing light L from the exposing means 3 depending on the image information.
The exposing means 3 here has a light source 3a such as a laser, a six-sided polygon mirror 3b for raster scanning, a lens 3c for imaging, and a fold mirror 3d, etc.
If the developing apparatus 5 for a first, black color among the four developing apparatuses mounted on the rotary 22 is here opposed to the photosensitive drum 1, negatively charged black toner particles will be deposited onto the electrostatic latent image formed on the photosensitive drum 1 by the developing apparatus 5 for a first, black color, and developed as a developer image (toner image).
The intermediate transferring belt 9 is supported by multiple support axes consisting of a secondary transferring opposing roller 12, a drive roller 15, and a tension roller 16.
In the image forming apparatus shown in FIG. 5, as an example, the intermediate transfer belt 9 may be an endless plastic belt having a thickness on the order of 0.05 mm to 0.3 mm, and having a volume resistivity on the order of 107 to 1011Ω·cm with its resistance adjusted by carbon, ZnO, SnO2, TiO2 and other conductive fillers. In this case, materials of the plastic belt may, for example, include PVdF (Poly(vinylidene fluoride)), Nylon, PET (polyethylene terephthalate), polycarbonate and the like.
By the rotation of the drive roller 15 (shown in a direction indicated by an arrow R2 in FIG. 5), the intermediate transferring belt 9 rotationally runs, as shown in FIG. 5, in an opposing position to the photosensitive drum 1 and in a direction indicated by an arrow R3 in FIG. 5 that is oriented in the same direction as the drum. When a primary transferring roller 10 having a diameter of 12 mm, which is located to be opposed to the photosensitive drum 1 across the intermediate transferring belt 9 and rotated following the intermediate transferring belt 9, is applied with a positively charged, primary transferring bias by a primary transferring power supply 20, the toner image on the photosensitive drum 1 is primarily transferred through a primary transferring nip N1 provided as a primary transferring position.
Here, as an example, +500 V DC voltage is used for the primary transferring bias. The primary transferring roller 10 typically consists of a material such as EPDM, urethane rubber, CR or NBR of which the volume resistivity is adjusted by adding a resistance controlling agent, such as carbon.
After the primary transfer, residual toner in the primary transfer is removed from the surface of the photosensitive drum 1 by the photosensitive drum cleaner 19 with a resilient blade.
A sequence of the afore-mentioned image forming processes of charge, exposure, development, primary transfer, and cleaning are sequentially repeated for each color toner of a second, magenta, a third, cyan, and a forth, yellow contained in developing apparatuses 6, 7 and 8 respectively, moved to an opposing position to the photosensitive drum 1, in order to layer toner images in four colors onto the intermediate transferring belt 9.
In an apparatus shown here, a primary transferring bias of +500 V is used for each color from the first to the fourth.
When a secondary transferring roller 11 having an outer diameter of 20 mm, which is located to be opposed to a secondary transferring opposing roller 12 rotating in a direction indicated by an arrow R4 in FIG. 5 that is oriented following the rotation of the intermediate transferring belt 9 across the intermediate transferring belt 9, is applied with a secondary transferring bias by a secondary transferring power supply 21, the four-color layered toner image on the intermediate transferring belt 9 is secondarily transferred at a time onto a surface of a transfer material P at a secondary transferring nip N2 of the secondary transferring opposing roller 12 and the secondary transferring roller 11, provided as a secondary transferring position. The intermediate transferring belt 9 is therefore a movable belt for transferring a toner image on the photosensitive drum 1 onto a transfer material. As an example, +1.5 KV is used for the secondary transferring bias.
The transfer material P bearing a four-color, unfixed toner image on its surface is conveyed to a fixing apparatus, not shown, in which the toner image on the surface is fixed to complete an image forming process.
After the secondary transfer as described above, the secondary transfer residual toner that consists of residual developer left untransferred on the intermediate transferring belt 9 is positively charged by a residual toner charging roller 123 that is applied with a positive DC voltage with a residual toner charging power supply 113 as means for charging residual developer, as described in Japanese Patent Application Laid-Open No. 9-44007.
The residual toner charging roller 123 has a mechanism, not shown, that is in spaced and abutted in relation to the intermediate transferring belt 9, and abuts against the belt only when it is charged. A reverse side of an abutting area N13 where the residual toner charging roller 123 and intermediate transferring belt 9 abut against each other is provided with a grounding opposing electrode 124 for increasing charging efficiency.
Finally, the positively charged toner is electrostatically transferred to the photosensitive drum 1 at the primary transferring nip N1, and the secondary transfer residual toner left on the intermediate transferring belt 9 is removed. Also, residual toner transferred to the photosensitive drum 1 is then removed by the photosensitive drum cleaner 19.
As another example of the means for removing the secondary transfer residual toner, the residual toner charging roller 123 may be applied with a bias voltage that is an alternating voltage superimposed by a positive DC voltage, as described in Japanese Patent Application Laid-Open No. 11-161043.
When means in which the secondary transfer residual toner is charged and electrostatically transferred to the photosensitive drum 1 is used as means for removing the secondary transfer residual toner, an adequate level of cleaning cannot be provided without a generally uniform amount of each charge of toner particles in the charged secondary transfer residual toner.
Toner particles with a lower amount of charge on the intermediate transferring belt 9 may be subject to a weaker electrostatic force from an electric field formed at a primary transferring nip N1, thereby failing to be transferred to the photosensitive drum 1. Toner particles with a higher amount of charge may, on the other hand, be subject to a greater mirroring force from the intermediate transferring belt 9, thereby failing to be transferred to the photosensitive drum 1.
An amount of charge retained by each particle of the secondary transfer residual toner can be almost leveled by a DC voltage. When an apparatus is used in an environment of a high temperature and humidity, however, the secondary transfer residual toner is poorly charged, and an amount of charge retained by each toner particle of the secondary transfer residual toner cannot be leveled by a DC voltage.
Using a bias that is an alternating voltage having a DC voltage superimposed on it, which has an advantage over a DC voltage in terms of an electrostatic force, therefore, enables to almost level an amount of charge retained by each toner particle of the secondary transfer residual toner even when a large amount of the secondary transfer residual toner presents in an environment of a high temperature and humidity, thereby allowing to provide an adequate level of cleaning.
While a level of cleaning of the intermediate transferring belt increases, the toner scattering has been observed near the abutting area N13 of the residual toner charging roller 123 against the intermediate transferring belt 9, resulting in a contamination inside of a machine after actual printing using an image forming apparatus in which a bias voltage that is an alternating voltage having a DC voltage superimposed on it is applied to the residual toner charging roller 123 as means for charging residual toner.